1. Field of the Invention
The present invention relates to a transmission circuit for use in a communication apparatus for mobile telephony, wireless LAN or the like. More particularly, the present invention relates to a transmission circuit which can control an output power of a transmission signal quickly and accurately even when the transmission signal is output at a high modulation rate and in a wide dynamic range, and a communication apparatus comprising the transmission circuit.
2. Description of the Background Art
There is a demand for a communication apparatus for mobile telephony, wireless LAN or the like which can control an output power of a transmission signal quickly and accurately even when the transmission signal is output at a high modulation rate and in a wide dynamic range. Hereinafter, a conventional transmission circuit for use in such a communication apparatus will be described.
For example, Japanese Laid-Open Patent Publication No. 3-154430 (hereinafter referred to as Patent Document 1) discloses a transmission circuit which automatically suppresses an output power of a transmission signal to a predetermined set value with high-speed response even when burst waves are transmitted as a transmission signal. FIG. 16 is a block diagram illustrating a configuration of a conventional transmission circuit 500 described in Patent Document 1. In FIG. 16, the conventional transmission circuit 500 comprises a transmission power amplifying section 504, an antenna 506, a loop filter 510, a gain adjuster 511, a linear modulator 512, a monitor wave generator 513, a combiner 514, band-pass filters 515 and 516, and a level detector 517. The transmission power amplifying section 504 includes a variable attenuator 502 and a power amplifier 503. The gain adjuster 511 includes a error amplification circuit 508 and a power supply circuit 509.
The linear modulator 512 outputs linearly modulated waves by linear modulation. The monitor wave generator 513 generates monitor waves which have a frequency which is not included in a transmission band of the linearly modulated waves and does not have a variation in amplitude (i.e., not modulated). The combiner 514 combines the linearly modulated waves and the monitor waves, and outputs the combined signal. The combined signal is amplified by the transmission power amplifying section 504, and is output to the band-pass filter 515 and the band-pass filter 516. The band-pass filter 515 extracts only linearly modulated waves from the combined signal, and outputs the extracted linearly modulated waves as a transmission signal from the antenna 506.
The band-pass filter 516 extracts only monitor waves from the combined signal, and outputs the extracted monitor waves to the level detector 517. The level detector 517 converts a level of the monitor waves into a detection voltage corresponding to the level of the monitor waves, and inputs the detection voltage to one of two input terminals of the error amplification circuit 508. A reference voltage Vn is input from the power supply circuit 509 to the other input terminal of the error amplification circuit 508. The error amplification circuit 508 amplifies an error voltage which is a difference between the detection voltage of the monitor waves and the reference voltage Vn. The amplified error voltage is fed back to the transmission power amplifying section 504 via the loop filter 510. The transmission power amplifying section 504 controls again for amplifying the combined signal, based on the fed-back error voltage. Thus, the conventional transmission circuit 500 has a closed loop for a power control to automatically suppress the output power of a transmission signal to the predetermined set value.
Japanese Laid-Open Patent Publication No. 6-284022(hereinafter referred to as Patent Document 2) discloses a conventional transmission circuit which opens and closes a loop for a power control to change a response speed of the power control. FIG. 17 is a block diagram illustrating a conventional transmission circuit 600 described in Patent Document 2. In FIG. 17, the conventional transmission circuit 600 comprises a power amplifier 601, a coupler (directional coupler) 602, a detection circuit 603, an antenna 604, a gate circuit 605, a integration hold circuit 606, a error amplifier 607, an analog switch 608, a voltage control attenuator 609, a buffer amplifier 610, a voltage adjusting section 614, and a correction section 623.
The transmission circuit 600 outputs an input modulated signal as a transmission signal from the antenna 604 via the voltage control attenuator 609, the buffer amplifier 610, the power amplifier 601, and the coupler 602. In the transmission circuit 600, the detection circuit 603, the gate circuit 605, the integration hold circuit 606, the error amplifier 607, and the analog switch 608 are provided at an output of the coupler 602 to form a closed loop for controlling a gain of the voltage control attenuator 609. The correction section 623 stores a set reference value A which is used during a burst operation with the same transmission power, and a set reference value B which is used immediately after changing of the output power.
During a burst operation with the same transmission power, the transmission circuit 600 generates a reference voltage A based on the set reference value A, inputs the reference voltage A to the error amplifier 607, and adjusts the gain of the voltage control attenuator 609 using the closed loop. On the other hand, immediately after changing of the output power, the transmission circuit 600 switches the analog switch 608 to open the closed loop, generates a reference voltage B based on the set reference value B, inputs the reference voltage B to the voltage adjusting section 614, and adjusts the gain of the voltage control attenuator 609 using the open loop. Thus, the conventional transmission circuit 600 opens and closes the loop for a power control to change the response speed of the power control.